Every style is converted into a unique number. This code is getting the number for the style Rectangle.

The same is true for pieces. To get the piece's number we use

@Piece[Base]

Understanding that we can look at the .mac file and find the following code.

if ( Flow[ 0 ] == 2 )

This is straight C code testing if an array value is equal to a constant. The key here is the options chosen when running the macro populates the Flow array. There is one element for each piece. Flow[0] corresponds to the first Piece (Base). If we added another piece it would have the value 1.

The value of Flow[0] contains the unique value of a style. There are 3 styles in Base. They are:

Base

Triangle

Rectangle

While it is possible to hard code the values it quickly becomes confusing. Accessors make the code more readable.